The eight key Blackrock Gems are found on the other worlds in Ultima Underworld II. They are by-products of the Guardian's spell to create the Blackrock Dome, and are also its weakness. The Avatar found the Gems, and after a treatment from Nystul, used them on the big Blackrock Jewel in the sewers, to weaken it and open passages to other worlds. These steps were essential to destroying it in the end. The Gems were: